1. Field of the Invention
This invention relates to a high pressure discharge lamp, and more particularly, to an apparatus and a method of producing a high pressure discharge lamp that is used as a light source in an automobile headlight or a fog light.
2. Discussion of the Related Art
A horizontal cross sectional view of a conventional high pressure discharge lamp is shown in FIG. 3. The high pressure discharge lamp may be used in an automobile headlight. The high pressure discharge lamp 90 comprises a bulb 91 having a center portion 91a defining a discharge chamber filled with rare gas and metal halide, sealed portions 91c, and sealed ends 91b. The sealed ends 91b include a pair of electrodes 92 projecting a predetermined distance into the discharge chamber, molybdenum foils 93, and lead wires 94.
A conventional method to form the discharge chamber of the bulb 91 comprises the steps of providing a pipe, heating and softening its center portion 91a, and blowing air into the pipe so that the center portion 91a expands to be substantially barrel-shaped. In an actual application, an outer bulb and a socket are created as well, although the composition and steps are not discussed herein.
The conventional high pressure discharge lamp 90 has the following problems. First, the curvature of the surface between the center portion 91a and the sealed ends 91b greatly changes at the sealed portions 91c. Accordingly, during operation of the automobile headlight, spot-shaped glare light is emitted from the sealed portion 91c due to the curvature of the surface. This glare light creates the appearance that the headlight has two or more light sources. Second, since the curved surface of the discharge chamber, corresponding to the center portion 91a, is formed by blowing air into the pipe, it is difficult to mass produce bulbs with uniform dimensions. Finally, since the diameter of the discharge chamber is the largest around the center of the arc A, the arc A is prone to curvature, which causes an unfavorable light distribution pattern change over time.